In the past, an electronic circuit unit insertion and removal mechanism for inserting and removing an electronic circuit unit into and from a shelf has been known (see, for example, Japanese Laid-open Patent Publication No. 2008-53749). According to this insertion and removal mechanism, the shelf includes a groove section, the electronic circuit unit includes a rotatable lever provided with an engaging section at the tip thereof, a rotatable stopper that is connected to the lever and is provided with an engaging section at the tip thereof, and a housing section that houses the lever and the stopper and is provided internally with an engaging surface that may engage the engaging section of the stopper, the shelf is locked to the electronic circuit unit as a result of the lever being rotated and the engaging section of the lever engaging the groove section of the shelf, and, even when an unlocking force acts on the electronic circuit unit, the engagement between the engaging surface of the housing section and the engaging section of the stopper is not released.